|
Over
the last two hundred years or so in which evolutionary ideas have been
seriously proposed we can find a full spectrum of opinions on evolution and
progress. As philosopher Michael Ruse has observed “evolutionists, including
Darwinian evolutionists, are badly split on the question of whether or not the
path of evolution is progressive, from simple to complex, from blob to the
human.”
Writing
in the Origin of Species Darwin had
spoken of a “Tree of Life” formed
by the branching of species, with twigs and leaves reaching skyward. As we saw
earlier, Gould has championed an alternate image of a bush, as he believes that
the tree image incorrectly implies there is a degree of upward directionality
within evolution. With Darwin’s model it is easy to suppose that the uppermost
branches and twigs of the tree are superior to those further down. By contrast,
Gould wishes to emphasise the non-directionality, suggesting that we view life
as “a copiously branching bush,” with some branches near the top, and some at
the bottom, all being “continually pruned by the grim reaper of extinction,
[and] not a ladder of predictable progress.”
Gould
has had to defend his commitment to non-directionality against the observation
that more complex organisms have, in fact, been preceded by less complex ones.
In his classic text On Evolution,
John Maynard Smith offered a rather ambiguous response to this dilemma: “It is
in some sense true,” he writes, “that evolution has led from the simple to the
complex ... I do not think that biology
has at present anything very profound to say about this.” Maynard
Smith’s conclusion is justified if we consider natural selection in abstract
algorithmic terms. At this level of description we can expect to see an
increase in fitness, but this does not automatically imply an increase in
complexity since a complex organism is not necessarily more fit.
Perhaps
the most optimistic advocate for progress and directionality in evolution was
Jesuit palaeontologist Pierre Teilhard de Chardin. Writing in The Phenomenon of Man, first published
after his death in 1955, he proposed that a trend was visible in evolutionary
history which he referred to as the ‘law of increasing complexity
consciousness.’He proposed that this law, which has brought about human consciousness, will
continue influencing evolution into the future, culminating in what he called
the ‘omega point.’ As we might expect, Gould was severely critical of Teilhard,
describing any directionality as a “noxious, culturally embedded, untestable,
nonoperational, intractable idea that must be replaced...” Ruse
has noted that Gould’s position may have been in part due his abhorrence at the
possibility that science could be used to justify the claim that one species or
race is further evolved and so superior to another. Ruse is an even handed
philosopher who is not at all sympathetic to the ideas of Teilhard, in fact he
has characterised evolution as “going nowhere ... and rather slowly at that” but
also allows that “we might suppose that evolution, even Darwinian evolution, is
a lot more directed than someone like Gould allows.”
How
might Conway Morris’s work on convergence play into the question of
directionality? Once again fitness landscapes can help us visualise the
situation. While at first glance evolutionary convergence appears to introduce
a directionality that Gould would find repugnant, this is not the case.
Convergence implies that life will inevitably draw from a fixed set of fitness
strategies, but it does not tell us which of these strategies is superior. If
we picture the situation on a fitness landscape (viewed at the largest scale)
we can imagine a peak for bacteria-style fitness (i.e. highly robust, massive
growth rate, etc.) and a different peak for human-style fitness (i.e. fragile
body, the ability to respond intelligently to threats and opportunities). Neither
is more ‘natural’ than the other, nor is one superior to the other.
One
more parameter needs to be added to our model to allow it to represent the full
array of options; population density. This is tightly related to the time
parameter since the longer evolution runs, the greater the number of organisms.
The effect of this parameter is more easily visualised if we invert the
landscape such that the lower points now correspond to greater fitness to that
local environment. Organisms can be represented by ball bearings dropped at
random locations at a specific rate onto the landscape. In the inverted
landscape, natural selection acts just like gravity in physical landscapes,
drawing the ball bearings into valleys and basins.
The
parameters that influence the character of evolution are: the shape of the
fitness landscape (flat or highly contoured), the source of the fitness
landscape (arbitrary, planetary, or universal) and the population density (i.e.
the time allowed for evolution). The question of directionality in Earth’s
biological history depends very much on the kind of evolutionary scenario we
are actually in. The noteworthy options are as follows.
Weak
convergence
On
a weakly convergent (i.e. mostly flat) landscape the density factor does not
significantly affect the result. No part of the landscape is particularly
noteworthy, and we always find the entire landscape populated at random. In
this case, there is no overall directionality.
Strong
convergence, high population density
However,
for cases where convergence is significant, or if biology is held to be
universal, landscape density is critical. If 100 balls are dropped into a
landscape that has 9 major basins, it is likely that all of the basins will be
occupied. In this scenario, there is also no directionality per se, because the
landscape is likely to be ‘flooded.’ In real terms, this could mean that, yes,
human-like organisms and bacteria-like organisms were likely from the very
beginning, but now that they have evolved, evolution has reached a kind of
stasis. This is essentially the view described by Conway Morris: “Darwinian
evolution remains central as the agency,” he says, “but the nodes of occupation
are effectively predetermined from the big bang.”
Strong
convergence, low population density
On
a highly convergent but sparsely populated landscape, organisms may not yet be
at their optimum fitness. Crucially, there may be entire basins that have not
yet been occupied at all, but which will be in time; i.e. if only 9 balls have
been dropped into a landscape with ten major basins, one of them will be
unoccupied. This last scenario could be considered a directionality of sorts,
since evolution has ‘somewhere significant to go.’ This conception of
directionality is very different from Teilhard’s omega point, and does not
appear to be the kind that Gould would find repulsive.
Printer-friendly
| Feedback | Contributed by: Adrian Wyard
|
